(a) Technical Field
The present disclosure relates, in general, to a method for preparing a cross-linked ceramic-coated separator containing an ionic polymer, a ceramic-coated separator suitably prepared by the method, and a lithium secondary battery using the same. In particular preferred embodiments, the present invention relates to a method for preparing a cross-linked ceramic-coated separator in which a coating material containing ceramic particles for improving thermal and mechanical characteristics, a functional inorganic compound for improving cycle characteristics and high rate characteristics of a battery, and an ionic polymer for bonding the ceramic particles and the functional inorganic compound on a porous membrane substrate is suitably coated on the porous membrane substrate and then subjected to chemical cross-linking, a ceramic-coated separator suitably prepared by the method, and a lithium secondary battery using the same.
(b) Background
Recently, lithium secondary batteries have attracted much attention as medium and large power sources for power tools, electric bicycles (e-bike), hybrid electric vehicles (HEVs), plug-in HEVs, and the like, as well as power sources for portable electronic devices such as cellular phones, notebook computers, and the like.
Due to the expansion of application fields and the increase in demand, the external shapes and sizes of the batteries are varied, and the lithium secondary batteries are required to have durability and safety characteristics better than those of existing small batteries.
Preferably, the lithium secondary battery is preferably prepared in such a manner that materials capable of intercalating and deintercalating lithium ions are used as negative and positive electrode materials, a porous separator is suitably disposed between the two electrodes, and an electrolyte is suitably injected into the porous separator. Preferably, electricity is generated or consumed by oxidation and reduction reactions due to intercalation and deintercalation of lithium ions in the negative and positive electrodes.
Preferably, the porous separator serves to prevent physical contact between the negative and positive electrodes and, at the same time, allow lithium ions to pass through pores. While the separator itself does not participate in an electrochemical reaction during charge/discharge, it may have a considerable effect on the cycle characteristics and safety of the battery based on the porosity, hydrophilicity, and material quality.
Preferably, a polyolefin membrane used as the separator of the lithium ion battery suitably exhibits a thermal shrinkage behavior at a temperature over 100° C. due to material characteristics and manufacturing process characteristics including elongation, and thus a short circuit between the negative and positive electrodes suitably occurs at high temperatures, which may cause safety accidents. Further, in terms of mechanical characteristics, the polyolefin separator has suitably low physical strength, and thus an internal short circuit easily occurs due to foreign materials present in the battery.
Since the polyolefin separator exhibits hydrophobicity due to its material characteristics, the affinity for an electrolyte having high polarity is suitably low. Further, since it is difficult for the electrolyte to permeate the separator, the electrolyte may leak to the exterior of the battery.
Accordingly, it is necessary to develop a new separator capable of suitably maintaining the performance of the lithium secondary battery and suitably improving the thermal safety, the mechanical strength, and the affinity for the electrolyte, which will be able to suitably accelerate the development of lithium secondary batteries having high performance and high safety.
In accordance with this trend, extensive research and development on organic/inorganic composite separators that are suitably prepared by coating ceramic particles and a binder polymer on a polyolefin or polyester porous membrane have been carried out.
U.S. Patent Publication No. 20050255769, incorporated by reference in its entirety herein, is directed to a separator that is prepared by coating hydrophilic ceramic nanoparticles (such as aluminum oxide, silicon oxide, and zirconium oxide) on a sheet-like flexible substrate such as polyethylene terephthalate nonwoven having a thickness of less than 30 μm and preferably exhibiting excellent thermal and chemical characteristics using an inorganic binder material. The thus prepared separator is thermally stable at high temperatures, does not cause thermal shrinkage, and exhibits good electrolyte wettability.
Korean Patent No. 0775310, incorporated by reference in its entirety herein, is directed to an organic/inorganic composite porous separator preferably including an active layer formed by suitably coating the surface of a separator substrate with a mixture of inorganic particles and a binder polymer, and an electrochemical device including the same separator. Preferably, the electrochemical device including the organic/inorganic composite porous separator is suitably applied to a lithium secondary battery to improve thermal and electrochemical safety and performance of the lithium secondary battery.
When the ceramic materials are suitably coated on the separator substrate in the above-described manner, the thermal and mechanical characteristics are suitably improved to prevent the positive and negative electrodes from being short-circuited, thus maintaining the safety of the battery under severe conditions such as high-temperature and overcharge. However, it is necessary that a sufficient amount of ceramic particles be suitably contained in the ceramic materials coated on the porous separator to prevent thermal shrinkage and provide sufficient mechanical strength.
Since the adhesive strength between the coating materials (ceramic particles and binder polymer) and the porous separator substrate is suitably low, the coating materials are easily exfoliated from the substrate and serve as a resistance after manufacturing the battery, thus suitably deteriorating the battery performance.
Moreover, the coated ceramic particles may be suitably separated from the porous separator by the stress that is generated during the battery assembly process including winding, and the thus separated ceramic particles act as a local fault of the secondary battery, which may have a bad effect on the safety of the battery.
Especially, when the adhesive strength between the coating materials and the porous separator substrate is suitably low, the ability for preventing the substrate from thermally shrinking is reduced, and thus it is difficult to prevent a short circuit between the positive and negative electrodes when the lithium secondary battery is over heated.
Further, although the materials widely used as ceramic materials (such as, but not limited to, silica, aluminum oxide, and silicon oxide) contribute to suitably improve the mechanical characteristics of the separator, they serve as a resistance in terms of battery characteristics, thus suitably deteriorating the battery characteristics.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.